A special election in Florida this week was seen as sort of a weathervane for political winds. A little-known Republican captured the vacant congressional seat, largely by tying his opponent to the unpopular Affordable Care Act. It's a playbook Republicans will try to use across the country - including in Colorado, where the U.S. Senate race has just gotten a lot more competitive. NPR's Scott Horsley reports.

SCOTT HORSLEY, BYLINE: For weeks now, President Obama's been warning Democrats not to be complacent about this year's November elections. His party's been doing fine in presidential years, but many of those who voted for Obama don't bother showing up in off-year contests.

PRESIDENT BARACK OBAMA: A lot of Democrats don't vote during midterms. We just don't.

HORSLEY: That's what happened four years ago when the generally older, whiter voters who did turn out famously gave the president's party a shellacking. One exception was in Colorado, where Sen. Michael Bennet was the rare Democrat to win a contested race that year, thanks to a surprisingly strong showing among women and Latino voters. This year, Bennett's got the job of preserving Democrats' slim majority in the Senate. Finding and turning out hard-to-reach voters is a big part of his strategy. Craig Hughes managed Bennet's 2010 campaign.

CRAIG HUGHES: You can't just knock on somebody's door two days before the election and say please vote, because they don't know why they should vote. So making the contrast between the two candidates is the first phase of this. What's at stake here for you, for your family? How does your pocketbook - impacted by these races?

HORSLEY: Democrats are trying to replicate Bennett's 2010 success in nearly a dozen contested Senate races this year. The $60 million effort includes data-driven modeling, to identify would-be supporters who might ordinarily sit out the election, as well as an army of paid staff and volunteers making repeat contacts to nudge those people towards the polls.

HUGHES: The knocks on the door are critical, the phone calls, the repetition, making sure that they know how they can vote; just continually, day after day after day, reminding them what's at stake.

HORSLEY: Democrats call this mobilization effort the Bannock Street Project. The name comes from the address of Bennett's 2010 field campaign office. The one-story building, across the street from a medical marijuana center, used to hum with telephones and computer terminals. Today, it houses a high-impact fitness center.

HORSLEY: At first, Democrats didn't think they'd have to flex a lot of muscle in Colorado's own Senate race this year. The incumbent, Mark Udall, was considered a fairly safe bet to win re-election. That suddenly changed two weeks ago, when Republican congressman Cory Gardner jumped into the race. Former State GOP Chairman Dick Wadhams says for the first time in more than a decade, Colorado Republicans are putting their best foot forward.

DICK WADHAMS: Cory is the whole package. He attracts people. He does not repel them. (Laughter) And we've had too many candidates who seem to repel voters.

HORSLEY: In fact, Republicans spent much of last year trying to recruit Gardner to challenge Udall. But with a safe House seat of his own, Gardner repeatedly said no. He changed his mind after a poll in February showed Udall looking more vulnerable than expected. Political analyst Floyd Ciruli blames shrinking support for the health care law and for President Obama.

FLOYD CIRULI: It's not clear that it's a wave, but at least it is definitely a burden. The Affordable Care Act, the president's own numbers - sort of the brand of the Democratic Party, at the moment, is hurting Udall.

HORSLEY: Those same headwinds are blowing in the face of Democrats nationwide, creating an opening for Republicans to raise money and attract stronger candidates. Gardner seems well-suited to capitalize on anger over the health care law. Last fall, he publicly confronted Health Secretary Kathleen Sebelius when his own family's insurance plan was canceled.

(SOUNDBITE OF ARCHIVED RECORDING)

REP. CORY GARDNER: Here's my letter. This is the letter that my family got, canceling our insurance. The White House website says if you like your health plan you have, you can keep it. Did I hear it wrong?

HORSLEY: But former Bennet campaign manager Craig Hughes argues the election won't necessarily be a referendum on the president or the health care law. As unpopular as Obama is, House Republicans have even lower approval ratings. The Colorado insurance exchange has performed relatively well. And in a state where one out of seven eligible voters is Latino, Hughes says Gardner's opposition to immigration reform could be a liability.

HUGHES: That can, again, mobilize the Latino vote - not only keep turnout up, but the percentage of the vote that Democratic candidates have been getting with Latino voters has increased dramatically since 2004. And I think that is, again, something that can really push Sen. Udall over the top.

HORSLEY: Democrats immediately tried to paint Gardner as too conservative for mainstream Coloradans, a tactic that's worked in the past against less-polished candidates. Ciruli says even though Gardner does have a very conservative voting record, the affable politician has shown an ability to appeal to suburban swing voters.

CIRULI: Udall's going to throw everything in the kitchen sink at Gardner. They've got a playbook. He is going to be labeled as part of the war on women. He is very conservative on environmental issues. So we have a long way to go, here. But I do believe that if Republicans can still win in this state, that's the candidate that can do it.

HORSLEY: There will be a lot of door-knocking and phone-calling between now and November. But what had looked like a low-impact exercise for Colorado Democrats just a few weeks ago, has suddenly turned into a full-body workout.

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